correct `From<SmallVec<T, N, A>> for Box<[T]>` implementation

[alejandro-vaz]

the current implementation on conversions.rs:

impl<T, const N: usize, A: Allocator> From<SmallVec<T, N, A>> for Box<[T]> {
    fn from(this: SmallVec<T, N, A>) -> Self {
        Vec::from(this).into_boxed_slice()
    }
}

first converts it to a Vec and changes the allocator before converting it into a box in Global

the correct implementation should preserve the allocator and not make use of an intermediary Vec:

impl<T, const N: usize, A: Allocator> From<SmallVec<T, N, A>> for Box<[T], A>